Driving power (8)
I believe the answer is:
momentum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">driving power</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for momentum that I've seen before include "Impelling force" , "Impetus gained by a moving object" , "driving power" , "Impetus gained by movement, maybe downhill" , "Impulse" .)